If we have an expression containing integer powers of the symbol u0 and
we know that u0's numerical value is (or at least eventually will be) u0
= Pi/2, we know what happens if we write
expr /. {u0 -> Pi/2}
But what about the reverse case: we have an expression containing
various integer powers of Pi, and we want to replace all the explicitly
appearing Pi's by 2 u0. Writing
expr /. {Pi-> 2 u0}
doesn't seem to do it.
Is there a way?